Porous diaphragms

ABSTRACT

Porous diaphragms suitable inter alia for electrolytic cells are formed from an aqueous slurry or dispersion of polytetrafluoroethylene and a solid particular additive utilising water as lubricant in the sheet forming operation.

Vallance June 17, 1975 POROUS DIAPHRAGMS [75] Inventor: Christopher Vallance, Runcorn, England [73] Assignee: Imperial Chemical Industries Limited, London, England 22 Filed: Jan. 24, 1973 21 Appl. No.: 326,475

[30] Foreign Application Priority Data Feb. 4, 1972 United Kingdom ..5351/72 [52] 'U.S. Cl. 264/49; 156/77; 210/500; 264/127 [51] Int. Cl. C08f 47/08 [58] Field of Search 264/49, 127

[56] References Cited UNITED STATES PATENTS 2,846,727 8/1958 Bechtold 264/49 2,968,522 1/1961 White 264/127 2,997,448 8/1961 Hochberg 264/49 3,281,511 10/1966 Goldsmith 264/49 4/1967 Gore 264/127 3,407,249 10/1968 Landi 264/49 3,576,686 4/1971 Schmidle 264/49 FOREIGN PATENTS OR APPLICATIONS I 241,695 11/1962 Australia 4. 264/127 639,568 4/1962 Canada 264/127 961,494 6/1964 United Kingdom 264/ 127 1,081,046 8/ 1967 United Kingdom 264/49 9 Primary ExaminerRona1d W. Griffin Attorney, Agent, or Firm-Cushman, Darby & Cushman [57] ABSTRACT Porous diaphragms suitable inter alia for electrolytic cells are formed from an aqueous slurry or dispersion of-polytetrafluoroethylene and a solid particular additive utilising water as lubricant .in the sheet forming operation.

28 Claims, No Drawings POROUS DIAPHRAGMS This invention relates to the manufacture of porous diaphragms.

More particularly, the invention relates to the manufacture of porous diaphragms based on polytetrafluoroethylene.

One method of manufacturing such porous diaphragms comprises forming an aqueous slurry or dispersion of polytetrafluoroethylene and a solid particular additive such as starch, adding an organic coagulating agent such as acetone to said dispersion and then drying the coagulated dispersion. An organic lubricant such as petroleum ether is then added to the dried coagulated material to serve as a processing aid when the material is being rolled into a sheet. On completion of the rolling operation the starch is removed to give the desired porous diaphragm. The lubricant can also be removed if required.

A major disadvantage of the above method of manufacturing porous diaphragms is that the use of an organic lubricant gives rise to irreproducibility of the diaphragms and this is extremely undesirable particularly when the diaphragms are intended for use in multimodular electrolytic cells where reproducibility is essential for efficient operation. We have now discovered that the above disadvantage is obviated or mitigated by the method of the present invention in which water is used as lubricant which enables diaphragms of desired reproducibility and permeability to be produced.

According to the present invention a method ofmanufacturing a porous diaphragm comprises preparing an aqueous slurry or dispersion comprising polytetrafluoroethylene and a solid particulate additive, thickening said aqueous slurry or dispersion to effect agglomeration of the solid particles therein, forming from the thickened slurry or dispersion a dough-like material containing sufficient water to serve as lubricant in a subsequent sheet forming operation, forming a sheet of desired thickness from said dough and removing solid particulate additive from the sheet.

In one embodiment of the invention the thickening of the aqueous slurry or dispersion is effected by reducing the water content thereof and water is then added to the thus thickened material to form the dough.

The desired degree of lubrication for the sheet forming operation is obtained by mixing water with the thickened material so that a dough having a viscosity of at least 300 poises is obtained. Preferably, water is added to the thickened material so that a dough having a viscosity of between 1 and 7X10 poises is obtained.

Depending on the end use of the diaphragm, the desired degree of lubrication can be achieved either by drying the aqueous slurry or dispersion to a low water content and then adding a considerable amount of water to form the dough or conversely drying the slurry or dispersion only slightly and adding comparatively less water to form the dough.

When the diaphragms are intended for use in electrolytic cells the aqueous slurry or dispersion is preferably dried to a water content of no more than 10% of the total weight of the dried dispersion.

Further preferably, water is added to the dried dispersion until a dough is attained which has a water content comprising 2 to 50%, preferably to 45%, of the total weight of the dough.

The drying of the slurry or dispersion can be carried out in any suitable manner which will not cause damage to the constituents thereof. Preferably the drying is carried out at a temperature of 10 to C for example 15 to 50C. The time for drying will depend, inter alia, on the temperature but is generally from 10 to 100 hours, for example, 20 to 50 hours.

In a further embodiment of the invention the aqueous slurry or dispersion is thickened by subjecting it to high shearing action and the high shear action is continued so that a dough having a viscosity of at least 300 poises, preferably between l l0 and 7X10 poises, is obtained.

In this embodiment the slurry or dispersion advantageously has a water content comprising 2 to 50%, preferably 20 to 45% of the total weight of the dispersion.

A particularly suitable way of subjecting the slurry or dispersion to high shear conditions is to mix the slurry or dispersion in a Z-blade mixer.

When theslurry or dispersion is subjected to high shear conditions the viscosity increases and high shear action is preferably maintained for a time at least sufficient for the viscosity to reach the preferred range.

In yet a further embodiment of the invention the aqueous slurry or dispersion is thickened by first subjecting it to mixing action and then adding a thickening agent to achieve desired consistency for the sheet forming operation.

Preferably the thickening agent is a copolymer of maleic anhydride and an alkyl vinyl ether.

The particles size of the polytetrafluoroethylene in the aqueous slurry or dispersion is preferably in the range of 0.05 to 1 micron, e.g. 0.1 to 0.2 micron.

The solid particulate additive can be any which is substantially'insoluble in water but which can be removed by a suitable chemical or physical means which will not cause damage to the polytetrafluoroethylene. The additive may be starch, for example maize starch and/or potato starch, or a water-insoluble inorganic base or carbonate, for example calcium carbonate.

These additives-may be removed, for example, by soaking the sheet in an acid, preferably a mineral acid e.g. hydrochloric acid. Other additives which may be used include organic polymers which depending on the properties of the polymer may be removed from the sheet by dissolving with an organic solvent, by hydrolysis or by vaporisation. Mixtures of additives may be used and if necessary various treatments may be given to the sheet to remove the additive.

Generally, the additive has a particle size substantially all of which are within the range of 5 to 100 microns. The amount of additive will depend on the permeability desired in the final diaphragm. Thus, the weight ratio of additive to polytetrafluoroethylene may be, for example, from 10:1 to 1:10 preferably from 5:1 to 121.

In many cases itis desirable to incorporate other components in the aqueous slurry or dispersion which are not removed when the sheet is subjected to the treatment to remove the particulate additive. Examples of such components include particulate fillers generally inorganic fillers, for example, titanium dioxide which is particularly preferred, barium sulphate, asbestos, (e.g. amphibole or serpentine asbestos), graphite and alumina. Suitably the filler has a particle size of, for example, less than 10 microns and preferably less than 1 micron. The weight ratio of filler to polytetrafluoroethylene may be for example from :1 to 1:10,

preferably from 2:1 to 1:2.

In some cases it may be advantageous to add a coagulating agent, e.g. brine, to the dispersion to assist in the formation of the dough.

The sheet is generally formed from the dough by calendering. Preferably, the calendering is carried out by passing the dough through the rolls a number of times.

Generally, after some, or even after every pass through the rolls, the sheets are rotated through about 90 so that the calendering is carried out biaxially.

The diaphragms produced by the process of the present invention have a wide range of uses but are particularly suitable for use in electrolytic cells for electrolysis of alkali-metal halides, for the production of chlorine and caustic alkalis.

They are generally strong enough to be used without any support but for extra strength it may be desirable to incorporate a sheet of a suitable strengthening material, for example, a polymer gauze such as polypropylene gauze.

The process of the present invention enables the production without difficulty of a successive number of diaphragms over a period of time, each one having similar permeabilities. This is very necessary when using the diaphragms in electrolytic cells.

The invention is illustrated in the following Examples in which all parts and percentages are by weight.

EXAMPLE 1 To 100 parts of an aqueous dispersion of polytetrafluoroethylene containing 60% of the polymer in the form of particles approximately all in the size range 0.15 to 0.2 micron were added 101 parts of water, 60 parts of titanium dioxide of particles size approximately 0.2 micron, 60 parts of maize starch of particle size approximately 13 microns and 120 parts of potato starch of particle size less than 75 microns. The mixture was then stirred wtih a paddlemixer for 30 minutes to form a substantially uniform paste. This paste was spread on trays and dried at 24 for 48 hours to a water content 5.7% by weight. 100 parts of the resultant crumb were mixed with 52 parts of water to form a dough having a viscosity of 4 10 poise, The dough was then spread along the shortest edge of a rectangular piece of card, and calendered on the card between dual, even-speed, calender rolls, set 3 mm apart, into an oblong sheet. After calendering, the oblong sheet was cut, in the direction of calendering, into four equal pieces. These were laid congruently over each other to obtain a fourlayered laminate. The card was picked up, rotated 90 in the horizontal plane, and calendered (directed 90 to the original direction of calendering) again through the 3 mm roll separation. This process, the successive cutting into four, stacking, rotating and calendering was repeated until the composition had been rolled a total of five times. The resultant laminate was cut into four, in the direction of calendering, stacked, removed from the card, and calendered, without rotation through 90, the inter-roll space being reduced by the thickness of the card. After calendering, the laminate was cut, at right angles to the direction of calendering, into four equal pieces, stacked, rotated through 90 and calendered again. This process, cutting at right angles to the direction of calendering, stacking, rotating and calendering was repeated until the composition had been rolled a total of nine times. The resultant essentially rectangular laminate was then passed through the rolls with its largest side directed at 90 to the direction of calendering. and with the inter-roll space slightly reduced, no cutting, stacking or rotating through 90 being involved. This process was repeated through a gradually reduced inter-roll space, the same edge of the laminate being fed to the rolls on each occasion, until the thickness of the laminate was 1.5 mm. A square of 22 26 mesh gauze woven of 0.011 inch diameter monofilament polypropylene yarn was placed on top of the laminate, and rolled into the laminate by calendering through a slightly reduced inter-roll space. The resultant reinforced sheet was removed from the rolls and soaked in cold aqueous 18% hydrochloric acid for 24 hours. The starch additive was thereby removed leaving a multi-porous sheet suitable for use as a diaphragm material in electrolytic cells electrolysing aqueous solutions.

EXAMPLE 2 To 100 parts of an aqueous dispersion of polytetrafluoroethylene containing 60% of the polymer in the form of particles approximately all in the size range 0.15 to 0.2 micron were added parts of water, 60 parts of titanium oxide of particle size approximately 0.2 micron, 60 parts of maize of particle size approximately 13 microns and 120 parts of potato starch of particle size less than microns. The mixture was then stirred with a paddlemixer for 3 minutes to form a paste which was then mixed in a Z-blade mixer for 22 minutes to form a dough having a viscosity of 4X10 poise. The dough was then spread along the shortest edge of a rectangular piece of card, and calendered on the card between dual, even-speed, calender rolls, set 3 mm apart, into an oblong sheet. After calendering, the oblong sheet was cut, in the direction of calendering, into four equal pieces. These were laid congruently over each other to obtain a four-layered laminate. The card was picked up, rotated in the horizontal plane, and calendered (directed 90 to the original direction of calendering) again through the 3 mm roll separation. This process, the successive cutting into four, stacking, rotating and calendering was repeated until the composition had been rolled a total of 15 times. The resultant laminate was cut into four, in the direction of calendering, stacked, removed from the card, and calendered, without rotation through 90, to the inter-roll space being reduced by the thickness of the card. After calendering, the laminate was cut, at right angles to the direction of calendering, into four equal pieces, stacked, rotated through 90 and calendering again. This process, cutting at right angles to the direction of calendering, stacking, rotating and calendering was repeated until the composition had been rolled a total of nine times. The resultant essentially rectangular laminate was then passed through the rolls with its largest side directed at 90 to the direction of calendering, and with the inter-roll space slightly reduced, no cutting, stacking or rotating through 90 being involved. This process was repeated through a gradually reduced inter-roll space, the same edge of the laminate being fed to the rolls on each occasion, until the thickness of the laminate was 1.5 mm. A square of 22 26 mesh gauze woven of 0.011 inch diameter monofilament polypropylene yarn was placed on top of the laminate, and rolled into the laminate by calendering through a slightly reduced inter-roll space, The resultant reinforced sheet was removed from the rolls and soaked in cold aqueous 18% hydrochloric acid for 24 hours. The starch additive was thereby removed leaving a multiporous sheet suitable for use as a diaphragm material in electrolytic cells electrolysing aqueous solution.

EXAMPLE 3 To 100 parts of an aqueous dispersion of polytetrafluoroethylene containing 60% of the polymer in the form of particles approximately all in the size range 0.15 to 0.2 micron were added 200 parts of water, 60 parts of titanium dioxide of particle size approximately 0.2 micron, 60 parts of maize starch of particle size approximately 13 microns and 120 parts of potato starch of particle size less than 75 microns. The mixture was then stirred with a paddlemixer for 2 minutes to form a substantially uniform paste having a viscosity of 3 poise. To this paste was added 27 parts of a thickening agent known at Viscofas L100 (a copolymer ofmaleic anhydride and methyl vinyl ether) and 27 parts of M NaOI-I, and the mixture stirred with a paddlemixer to yield a dough having a viscosity of l l0 poise. The dough was then spread along the shortest edge of a rectangular piece of card, and calendered on the card between dual, even speed, calender rolls, set 3 mm apart, into an oblong sheet. After calendering, the oblong sheet was cut, in the direction of calendering, into four equal pieces. These were laid congruently over each other to obtain a fourlayered laminate. The card was picked up, rotated 90 in the horizontal plane, and calendered (directed 90 to the original direction of calendering) again through the 3 mm roll separation. This process, the successive cutting into four, stacking, rotating and calendering was repeated until the composition had been rolled a total of one hundred and ten times. For the first ninety of these passes through the rolls, accurate stacking into laminates was not possible due to the nature of the material. The resultant laminate was cut into four, at'right angles to the direction of calendering, stacked, removed from the card, rotated through 90, and calendered, the inter-roll space being reduced by the thickness of the card. This process, cutting at right angles to the direction of calendering, stacking, rotating and calendering was repeated until the composition had been rolled a total of one hundred and fifteen times.

The resultant essentially rectangular laminate was then passed through the rolls with its largest side directed at 90 to the direction of calendering, and with the interroll space slightly reduced, no cutting, stacking or rotating through 90 being involved. This process was repeated through a gradually reduced inter-roll space, the same edge of the laminate being fed to the rolls on each occasion, until the thickness of the laminate was 1.5 mm. A square of 22 26 mesh gauze woven of 0.011 inch diameter monofilament polypropylene yarn was placed on top of the laminate, and rolled into the laminate by calendering through a slightly reduced inter-roll space. The resultant reinforced sheet was removed from the rolls and soaked in cold aqueous 18% hydrochloric acid for 24 hours, The starch additive was thereby removed leaving a multiporous sheet suitable for use as a diaphragm material in electrolytic cells electrolysing aqueous solutions.

In the process of the invention, high viscosity measurements were preferably made with a Weissenberg Rheogoniometer, Model R16 (Sangamo Controls Limited). Thisinstrument is equipped with a cone and plate sample assembly of 2.5 cm diameter with a cone angle of 4. The cone and plate are manufactured of stainless steel and the surfaces are polished. For viscosities below 5X10 poise, the torsion bar torque measuring accessories were used and the cone was driven in the continuous mode. For viscosities above 5X 10 poise the cone was driven in the oscillatory mode and the torque was measured by means of two type 9203 piezoelectric force transducers (Kistler Instruments Limited). set at 4 cm apart. The resulting output signal was compared with the input signal by means of analysers JM1606 and JX1600A (Solatron Limited). Viscosity measurements detailed are initial readings at a shear rate of y=l see.

For viscosity measurements below 600 poise however, for example as in the first viscosity measurement of Example 3, a Brookfield Viscometer, Model LVF was used. This instrument was equipped with a spindle number 2 and measurements quoted were initial measurements taken at speed 30.

We have also conducted tests to show that diaphragms prepared according to the methods of the present invention can be obtained with a greater degree of reproducibility than diaphragms obtained by means of an organic lubricant.

TEST SERIES A Table I Diaphragm No. K perm K perm. (i.c. permeability measured in hrs) flow rate through diaphragm (litres/hour) X 10 area of diaphragm (cm X pressure drop (cms, of brine) In Test Series A the diaphragm was installed in the electrolytic cell and the inorganic filler removed by operation of the cell. The permeability of the diaphragm is zero at first, increases as starch is removed and then reduces as the diaphragm becomes blocked during extended operation of the cell. The coefficient of permeability (K perm) is based on measurements taken when the permeability of the diaphragm is at a maximum. Comparison of the permeability factors indicated in Table I for diaphragms produced by identical techniques showed a wide deviation as regards reproducibility.

TEST SERIES B In this test series a number of diaphragms was prepared according to the process described in Example 1 except that petroleum ether was used as lubricant instead of water. In this case the inorganic filler was extractedoutside the electrolytic cell by soaking in 16% hydrochloric acid and permeability measurements were made when the diaphragms had been installed in the electrolytic cell. The results are shown in Table II below.

Table ll Diaphragm No.

Means K pcrm Standard Deviation These results indicate use of an organic lubricant gives rise to considerable irreproducibility of the diaphragms.

TEST SERIES C In this series of tests the diaphragms were prepared according to Example 1 with water being used as lubricant. This time a slightly different technique was adopted in measuring permeability in that a special rig was set up for removal of the inorganic filler by acid extraction and the permeability measurements were not made with the diaphragms installed in an electrolytic cell but in said rig. The tests results are indicated in Table III below which also indicates results of tensile strength tests carried out on the diaphragms on a standard lnstron tensile strength measuring device. We have established that the tensile strength of a diaphragm is correlated to its permeability and we have found that the tensile strength tests are easier to conduct and the results can be used to accurately predict The results listed in Table III indicate that by using water as lubricant diaphragms of desired reproducibility and permeability can be obtained.

What we claim is:

1. A method of manufacturing a porous diaphragm suitable for use in electrolytic cells comprising preparing an aqueous dispersion of polytetrafluoroethylene and a solid particulate removable filler, thickening said aqueous dispersion to effect agglomeration of the solid particules therein, forming from the thickened dispersion a dough-like material containing water as lubricant for a subsequent sheet forming operation, calendering said dough-like material to form a biaxially fibrillated sheet and removing the solid particulate filler from the sheet to render it porous.

2. A method as claimed in claim 1 wherein the thickening of the aqueous dispersion is effected by reducing the water content thereof and water is then added to the thus thickened material to form the dough.

3. A method as claimed in claim 2 wherein the desired degree of lubrication for the sheet forming operation is obtained by mixing water with the thickened material so that a dough having a viscosity of at least 300 poises at 20C is obtained.

4. A method as claimed in claim 2 wherein the drying of the aqueous dispersion is carried out at a temperature of 10 to C.

5. A method as claimed in claim 1 wherein said aqueous dispersion of polytetrafluoroethylene and a solid particulate removable filler is dried to a water content of no more than 10% of the total weight of the dried dispersion prior to the addition of further water for the formation of the dough.

6. A method as claimed in claim 1 wherein water is added to the dried dispersion until a dough is attained which has a water content comprising 20 to 45% of the total weight of the dough.

7. A method claimed in claim 1 wherein the aqueous dispersion is thickened by subjecting it to high shearing action and the high shear action is continued so that a dough having a viscosity of at least 300 poises at 20C is obtained.

8. A method as claimed in claim.7 wherein the dispersion has a water content comprising 2 to 50% of the total weight of the dispersion.

9. A method as claimed in claim 7 wherein the dispersion is subjected to high shear conditions by mixing it in a Z-blade mixer.

10. A method as claimed in claim 1 wherein the aqueous dispersion is thickened by first subjecting it to mixing action and then adding a thickening agent to achieve the desired consistency for the sheet forming operation.

11. A method as claimed in claim 10 wherein the thickening agent is a copolymer of maleic anhydride and an alkyl vinyl ether.

12. A method as claimed in claim 1 wherein the particle size of the polytetrafluoroethylene in the aqueous slurry or dispersion is in the range of 0.05 to 1 micron.

13. A method as claimed in claim 1 wherein the solid particulate removable filler is substantially insoluble in water and can be removed by chemical or physical means which does not cause damage to the polytetrafluoroethylene.

14. A method as claimed in claim 13 wherein the solid particulate additive is starch or a water-insoluble inorganic base or carbonate.

15. A method as claimed in claim 13 wherein the solid particulate filler is removed by soaking the sheet in acid.

16. A method as claimed in claim 1 wherein the solid particulate filler is an organic polymer removable by dissolving the sheet in an organic solvent or by hydrolysis or by vaporisation.

17. A method as claimed in claim 1 wherein the solid particulate removable filler has a particle size substantially within the range 5 to 100 microns.

18. A method as claimed in claim 1 wherein the ratio of solid particulate filler to polytetrafluoroethylene is :1 to 1:10.

19. A method as claimed in claim 1 wherein the aqueous dispersion contains a non-removable filler.

20. A method as claimed in claim 19 wherein the nonremovable filler is titanium dioxide, barium sulphate, asbestos, graphite or alumina.

21. A method as claimed in claim 20 wherein said nonremovable filler has a particle size less than 10 microns.

22. A method as claimed in claim 19 wherein the weight ratio of said non-removable filler to polytetrafluoroethylene is 10:1 to 1:10.

23. A method as claimed in claim 1 wherein a coagulant is added to the aqueous dispersion to assist in the formation of the dough.

24. A method as claimed in claim 23 wherein the coagulant is brine.

25. A method as claimed in claim 1 wherein the calendering is carried out by passing the dough rolls a 

1. A METHOD OF MANUFACTURING A POROUS DIAPHRAGM SUITABLE FOR USE IN ELECTROLYTIC CELLS COMPRISING PREPARING AN AQUEOUS DISPERSION OF POLYTETRAFLUOROETHYLENE AND A SOLID PARTICULATE REMOVABLE FILLER, THICKENING SAID AQUEOUS DISPERSION TO EFFECT AGGLOMERATION OF THE SOLID PARTICULES THEREIN, FORMING FROM THE THICKENED DISPERSION A DOUGH-LIKE MATERIAL CONTAINING WATER AS LUBRICANT FOR A SUBSEQUENT SHEET FORMING OPERATION, CALENDERING SAID DOUGH-LIKE MATEIAL TO FORM A BIAXIALLY FIBRILLATED SHEET AND REMOVING THE SOLID PARTICULATE FILLER FROM THE SHEET TO RENDER IT POROUS.
 2. A method as claimed in claim 1 wherein the thickening of the aqueous dispersion is effected by reducing the water content thereof and water is then added to the thus thickened material to form the dough.
 3. A method as claimed in claim 2 wherein the desired degree of lubrication for the sheet forming operation is obtained by mixing water with the thickened material so that a dough having a viscosity of at least 300 poises at 20*C is obtained.
 4. A method as claimed in claim 2 wherein the drying of the aqueous dispersion is carried out at a temperature of 10* to 100*C.
 5. A method as claimed in claim 1 wherein said aqueous dispersion of polytetrafluoroethylene and a solid particulate removable filler is dried to a water content of no more than 10% of the total weight of the dried dispersion prior to the addition of further water for the formation of the dough.
 6. A method as claimed in claim 1 wherein water is added to the dried dispersion until a dough is attained which has a water content comprising 20 to 45% of the total weight of the dough.
 7. A method as claimed in claim 1 wherein the aqueous dispersion is thickened by subjecting it to high shearing action and the high shear action is continued so that a dough having a viscosity of at least 300 poises at 20*C is obtained.
 8. A method as claimed in claim 7 wherein the dispersion has a water content comprising 2 to 50% of the total weight of the dispersion.
 9. A method as claimed in claim 7 wherein the dispersion is subjected to high shear conditions by mixing it in a Z-blade mixer.
 10. A method as claimed in claim 1 wherein the aqueous dispersion is thickened by first subjecting it to mixing action and then adding a thickening agent to achieve the desired consistency for the sheet forming operation.
 11. A method as claimed in claim 10 wherein the thickening agent is a copolymer of maleic anhydride and an alkyl vinyl ether.
 12. A method as claimed in claim 1 wherein the particle size of the polytetrafluoroethylene in the aqueous slurry or dispersion is in the range of 0.05 to 1 micron.
 13. A method as claimed in claim 1 wherein the solid particulate removable filler is substantially insoluble in water and can be removed by chemical or physical means which does not cause damage to the polytetrafluoroethylene.
 14. A method as claimed in claim 13 wherein the solid particulate additive is starch or a water-insoluble inorganic base or carbonate.
 15. A method as claimed in claim 13 wherein the solid particulate filler is removed by soaking the sheet in acid.
 16. A method as claimed in claim 1 wherein the solid particulate filler is an organic polymer removable by dissolving the sheet in an organic solvent or by hydrolysis or by vaporisation.
 17. A method as claimed in claim 1 wherein the solid particulate removable filler has a particle size substantially within the range 5 to 100 microns.
 18. A method as claImed in claim 1 wherein the ratio of solid particulate filler to polytetrafluoroethylene is 10:1 to 1:10.
 19. A method as claimed in claim 1 wherein the aqueous dispersion contains a non-removable filler.
 20. A method as claimed in claim 19 wherein the nonremovable filler is titanium dioxide, barium sulphate, asbestos, graphite or alumina.
 21. A method as claimed in claim 20 wherein said nonremovable filler has a particle size less than 10 microns.
 22. A method as claimed in claim 19 wherein the weight ratio of said non-removable filler to polytetrafluoroethylene is 10:1 to 1:10.
 23. A method as claimed in claim 1 wherein a coagulant is added to the aqueous dispersion to assist in the formation of the dough.
 24. A method as claimed in claim 23 wherein the coagulant is brine.
 25. A method as claimed in claim 1 wherein the calendering is carried out by passing the dough rolls a number of times.
 26. A method as claimed in claim 25 wherein after some or after every pass through the rolls, the sheets are rotated through about 90* so that the calendering is carried out biaxially.
 27. A method as claimed in claim 1 wherein the diaphragm is provided wtih a strengthening support.
 28. A method as claimed in claim 27 wherein the support is a polypropylene gauze. 